Malignant phenotypes of breast cancer cells can be reversed into normal cell phenotypes by embryonic environments, highlighting a possible novel cell-converting strategy to treat malignant breast cancers instead of killing cancerous cells. The induced Pluripotent Stem Cell (iPSC) technique can reprogram malignant cancer cells into Embryonic Stem Cell (ESC)-like cells, providing living embryonic environments. However, the low conversion of the current iPSC technique limits its broad applications in cancer research and treatment. We developed a protein-induced PSC (piPSC) technique that reprogrammed heterogeneous malignant Breast Cancer Cells (malBCCs) into piPSCs using the Sox2/Oct4/Nanog (SON) proteins with 904% conversion. Implantations of the 4T1-piPSCs into mouse mammary glands resulted in tumor stasis, metastasis inhibition and differentiation into normal mammary cells. Co-cultures of 4T1 cells with 4T1-piPSCs suggest a bystander effect that enables a small number of 4T1-piPSCs to reverse malignant phenotype of large numbers of surrounding 4T1 cells. Direct intra-tumor injections of the SON proteins in the 4T1 tumor bearing mice resulted in tumor stasis and metastasis inhibition, suggesting an in situ cell-converting cancer therapy. Tail vein injections of QQ-SON proteins targeted these proteins into the nuclei of tumor cells of breast cancer. Our central hypothesis is: Injections of the SON proteins convert malBCCs into piPSCs in situ that reverse malignant phenotype of the surrounding malBCCs into normal cells via a bystander effect of piPSCs. We will first optimize piPSC generation from mammary epithelial cells and malBCCs. Co-cultures of malBCCs with piPSCs will be performed to study if oncogenic properties of the co-cultured malBCCs can be reduced in vitro (Aim 1). To investigate the bystander effect in vivo, we will administer different doses of (small numbers) piPSCs into the tumor (large numbers) of the tumor-bearing mice at different time points to study if these piPSCs can induce tumor stasis and inhibit metastasis in vivo (Aim 2). To mimic the clinical setting of breast cancer treatment, we will directly inject the SON proteins either via tail vein or via intra-tumor into the breast cancer-bearing mice at different time points to investigate if the in situ cell conversion can induce tumor stasis and inhibit metastasis, therefore delaying breast cancer recurrence and prolonging survival (Aim 3). Different from the current molecular-based cell-killing therapies, we will generate tissue-specific piPSCs in situ as living embryonic environments to reverse malignant phenotype of breast cancer cells into normal cells. This will be achieved by injections of the SON proteins via tail vein and via intra-tumor into breast cancer metastatic mouse models. Although this proposal focuses on a proof-of-principle verification, a protein-induced in situ cell-conversion strategy wil be easily translatable to human clinical applications. The success of this proposal may set a stage for future human clinical trials of this proposed cell converting breast cancer therapy by significantly delaying cancer recurrence, prolonging survival and improving outcomes of breast cancer patients.